1. Field of the Invention
The present invention relates to external cooling of high-power piezoelectric transducers and, more particularly, toward a heat conducing layer for cooling piezoelectric transducers.
2. Description of the Related Art
The world industry produces a large number of standard piezoelectric transducers. A piezoelectric transducer is a device which converts high-frequency alternating electrical current to mechanical vibrations. High voltages (up to several kilovolts) and high currents (up to several amperes) are commonly supplied to its piezoceramic element rings. This requires the internal area of a transducer to be free from moisture or any other contamination that may lead to electrical arcing.
It is well-known that substantial heat is released during the operation of a piezoelectric transducer, which, unless removed, worsens the transducer's performance and ultimately destroys it. Heat generation relates to the dielectric properties of the transducer's piezoceramic elements as well as to the internal friction present in the transducer during operation. Efficient cooling of a piezoelectric transducer is, therefore, essential. In addition, maintaining a low working temperature of a transducer is known to significantly improve its operational parameters, make it more reliable and durable and increase its maximum output power limit.
The most common method of cooling a piezoelectric transducer is by blowing a cool, dry gas (commonly air) through its interior area. This method has several severe limitations. Due to its low heat capacity, very high air flow rates are required to cool transducers during continuous operation. At high powers, even very substantial flow rates may not be able to ensure stable and low operating temperature. The preparation of dry and cool air requires bulky equipment, such as a high-volume compressor and a desiccant inline dryer/filter. Any condensation in the tubes may be brought into the transducer by the air and cause irreversible damage. The housing of an air-cooled transducer must have an inlet and an outlet. Since the transducer is, therefore, not sealed to the environment, vapors or powders may reach its internal area where high-voltage contacts are located. Consequentially, when used in processes involving flammable or explosive materials, such as organic solvents or powders, the entire device becomes a potential explosion and fire hazard.
Several methods and designs of externally-cooled piezoelectric transducers have been suggested. In all of these methods, the transducer is placed in a sealed housing, which is equipped with a device for heat removal, such as a heat sink. For example, U.S. Pat. No. 4,169,387 by Krempl discloses a piezoelectric transducer cooled by a heat pipe system located within its housing and having mechanical contact with the transducer's surface. Cooling occurs through heat conduction created by evaporation and subsequent condensation of a liquid contained within the heat pipe system. A major disadvantage of this cooling method is its applicability only to low-power, low-amplitude transducers, such as piezoelectric pressure sensors. In the case of ultrasonic transducers, which operate at higher amplitudes, the mechanical contact of the heat pipe system with the transducer's surface and/or the system itself is likely to break.
U.S. Pat. No. 8,004,158 by Hielscher describes a stack of several transducers forming a common assembly with internal channels filled with a pressurized coolant liquid. The liquid can also flow in the space between the surface of the assembly and its housing. The heat generated by the assembly is dissipated by convection. This method's major disadvantage arises from the property of liquids brought in contact with vibrating transducer surfaces to undergo cavitation and absorb substantial amounts of acoustic energy. This sharply increases parasitic heat generation and lowers the transducers' efficiency. In addition, in this method there is a possibility of disrupting electrical contacts due to vibration-enhanced diffusion of the insulating liquid in between the transducer's piezoceramic element rings and electrodes.
In U.S. Pub. Pat. Appl. No. 2011/073,293 by Gauthier, a soft thermally-conductive wick is disclosed, which is connected between the metallic tail mass of a transducer and onto a thermal sink. Being woven from thin metallic wire strands with good heat conduction properties, the wick is soft, and neither adversely affects vibration properties of the transducer nor transmits vibrations to the thermal sink. This arrangement is quite useful for removing heat from the tail mass of a transducer; however, it does not permit heat dissipation from any location close to piezoceramic elements due to high voltages being present in that area. It is, however, essential to be able to cool the piezoceramic elements directly, since the heat is mostly generated in their ceramic material and their heat conduction properties are poor. If only the tail mass of a high-power transducer is cooled, local overheating and damage to the piezoceramic elements is likely to occur.
U.S. Pat. No. 6,481,493 by Hielscher describes a device designed to permit high-power ultrasonic transducers to operate for extended periods of time in environments with high heat and humidity. The device involves encapsulating the transducer's surface in a thin layer of silicone rubber, whose function is to electrically insulate the surface and absorb its vibrations. This “inner” layer is followed by another layer of silica sand, which is a thermally conductive material, whose function is to conduct the heat generated by the transducer to the inner wall of its sealed housing and ultimately to an external heat sink. There are several disadvantages to this “double-layer” approach. Silicone rubber is a poor heat conductor, having the thermal conductivity of only 0.15 W/m*K. Silica sand has a higher thermal conductivity (0.33 W/m*K for fine, dry powder), but is abrasive. In addition, the finely dispersed powder has a tendency to agglomerate over time, which lowers its thermal conductivity. In order to achieve sufficient rates of heat conduction from the transducer to the heat sink, it is, therefore, necessary to maintain the first, silicone rubber, layer as thin as possible (0.05-0.5 mm, as indicated in the patent). As such, the silicone rubber layer cannot completely absorb the vibrations, which results in its relative motion with respect to the silica sand layer. Due to the abrasive nature of the sand, overtime this motion results in the destruction of the silicone rubber layer, leading to the loss of electrical insulation and the possibility of arcing. If the moisture content of the sand increases as the housing vacuum seal is lost overtime, arcing may occur even if the thin silicone layer is still intact. A further disadvantage of this method is that cooling is only practically possible on the outside surface of the piezoceramic rings. Since the piezoceramic material has poor thermal conductivity, temperature gradients in the ceramics may result, causing fractures. In addition, the resulting average thermal conductivity of both layers is relatively low, which makes the cooling of high-power ultrasonic transducers during continuous operation quite challenging. Thus, this prior art method cannot fully enable continuous and safe operation of high-power piezoelectric transducers.
Based on the above, it can be concluded that an efficient and robust method for cooling high-power, high-amplitude piezoelectric transducers during continuous operation in unfavorable environments is still lacking, and an adequate and simple explosion and flame-proof cooling design for these types of transducers is not available in the prior art.